I'm planning to run RotRL (anniversary edition) soon. The party will probably consist of four players (as recommended by the book itself), all of them new to Pathfinder and possibly some new to tabletop RPGs in general, so I'll probably be giving them significant guidance in making their characters. I'm a fairly experienced player, and pretty familiar with the rules and character creation options (core rules and beyond), but an inexperienced GM. I've only had a chance to read the first two chapters in detail so far.

Could you please give me advice as to what sorts of characters, or what abilities, are necessary for this campaign? Which characters'/abilities' absence would make life cripplingly, or at least significantly, difficult for the party? And which, while they could be done without, would be likely to enhance the experience? I'm particularly (but not exclusively) interested in any thoughts on the following questions:

Could the party manage without an arcane full-caster?
Is it necessary to have a rogue or someone with certain rogue-ish abilities (e.g. to deal with traps and locks)?
Is it necessary to have a healer, and if so does it have to be a full-caster or would a paladin (or even a bard) do?
In a party of the recommended size of four, how many melee-focused or at least melee-capable characters are needed? How many archers, ranged support/utility types or other non-melee types is there room for in a party of that size?

Please let me know your thoughts on the above, and any other heplful thoughts you may have.

In case any of the following is relevant:

We'll probably be using 20-point buy for ability scores. As the players are new to the system I'll probably use only the core rulebook stuff as a starting point for helping them make their characters, but I'm open to introducing classes and archetypes from other sources if it fits their character concepts and/or the party's needs. Exception: I'd probably use the four Unchained classes over the core ones (for the PCs - NPCs will stay as written!), as I gather the consensus is that they're an improvement (but let me know if you think that's a bad idea). Definitely core races only.

No gunslingers, technology, Occult Adventures or non-Paizo stuff. I'd like to use the Background Skills rules from Unchained, and Skill Unlocks if there's a rogue, but I'm not thinking of using any other optional extra systems (e.g. Stamina, Hero Points) for fear of overwhelming the players (or myself!) - unless you think there are any whose exclusion would be a particular detriment?

Please excuse all that detail - it's not meant to suggest that I've made up my mind before listening to advice, merely to give you an indication of my current thoughts. All advice welcome!

An arcane full caster would be recommended, but not mandatory. They get a lot of useful utility and offensive spells that divine casters have less access to. Wizards would be especially good in this AP for figuring out all of the magical stuff that gets thrown at the party in the 2nd half of the AP.

Rogue necessary?

Spoiler:

The party can get by without a Rogue if no one wants to build one. Traps aren't prominent enough in this AP to justify building a Rogue on their own. Lock picking and being a skill monkey are the main reasons to play a Rogue in this AP. Bards or Rangers can easily fill this niche due to their generous skill points.

Healer necessary?

Spoiler:

Recommended. Wands and Potions can substitute for a healer.
But that will be expensive. Having a dedicated healer will be nice because the party will frequently be going into the wilderness/other dimensions where accessing an NPC cleric will take a hike.

How many melee, how many ranged?

Spoiler:

I'd go with one dedicated melee character, and one dedicated ranged character. The dedicated ranged character could be a sorcerer rather than an archer.

I'd personally outright replace the Core Barbarian and Monk with their Unchained counterparts if I were GMing for new players. The Unchained versions are a lot easier on beginners. But the Unchained Summoner may be a bit complicated for a beginner.

I'd definitely add the Oracle class to the allowed options so that there is a spontaneous Divine caster available to the players. That will make it easier for a beginner to play a healer.

I'll start with the basics - the system works best when all roles are filled: arcane, divine, skill, combat. If the pc's lack one role, they can overcome it but often the burden falls to the GM. Sometimes the gap can cause real problems and the GM has to adjust encounters to account for it.

You'll see options - example: the nice response from Emerald Cat - but those don't really change the need just how to fill it. Suggesting potions, wands, etc. to address healing doesn't change the fact that healing is needed, just how it's delivered. Ditto for bards or rangers filling the rogue "role."

Some AP specific comments:

Spoiler:

The AP really rewards a wizard, especially in the later books, when many of the main antagonists are wizards. Also the spell flexibility of a wizard gives the pc and group a lot of options but someone needs to be willing to take on some of the associated complexity of managing spellbooks, etc. Wizards make good magic item crafters which can be a way for the pc's to get items they might want - the AP provides an array of magic items but not necessarily a set that ideally fits each pc (which of course a GM can change.) Lastly some of the most important skills in the AP are Knowledge Skills which are the wizard's strength.

I'd agree with Emerald Cat that this AP at least does not absolutely call for a rogue, in that traps and the like are not a common or significant problem. Unchained Rogue is a good idea. Still some skill mastery will be of value.

It's one thing to use potions and wands and such for healing (though those cost money) but that only addresses hit point damage. It's a lot harder to solve other problems that call for divine magic - things that require spells like remove curse, restoration, break enchantment, heal, etc. And those start to appear much more regularly in the later books as the enemies become powerful spell-casters in their own right. Do have to have a cleric? No... But jiminy crickets will it be easier for the group to stay alive. And the players to have fun. "Oh, Bob, too bad. You failed your save and now you're totally incapacitated until we can get back to the city and find someone to fix you. Who knows when that will happen. You can sit there and do nothing for the next few hours until we find our way out of the dungeon."

You don't have to have a ranged attack character, though that's a good idea. Doing damage to the enemy before they get close to you is always a good military strategy. But there are a lot of ways to do that - wizards, sorcerers, rangers, rogues - with missile weapons, ditto for a bard. In this campaign in particular there are many sections with powerful melee enemies - ogres, giants, etc. - very good idea to take some of them out before they close and start smashing you.

Two other suggestions: read the whole AP through before you start character generation so you can see what I'm referring to in future books. And consider Hero Points. Nice solution to help players get out of sticky wickets without dying. Which until you get to higher level, have a lot of cash, have a cleric, etc. can be a hard and inconvenient problem solve. Including a headache for the GM - how do I incorporate a new pc for this player out in the middle of nowhere in the lost dungeon of antiquity that no one has found in a thousand years? (Hypothetical example.) I make some of these suggestions to help the GM as well as the players. When the pc's get stuck or TPK'd the GM suffers as well.

You can do it, but you shouldn't. The AP doesn't just reward arcane casters: towards the middle of the series it starts throwing magical resources at them at unprecedented levels. It does this because the end of the AP is a smackdown against an arcane caster that has all those resources and more.

Sure, you can do the AP without a full arcane caster, just as 3-legged dogs can still walk, and you can see with only one eye.

Arcane Caster - my 5 man party that went through RotR spent most of the AP with a Bard as their only arcane caster. He was a thundercaller bard, which added some arcane AoE damage, which I would recommend but it's not necessary. As others have said, there is a lot of Wizard loot in books 4-6.
One Note

Spoiler:

If your players lack a way to do magic damage/magic weapons in book 1 I recommend you remove the 2 shadows in the final dungeon.

Rogue - My party didn't have one. They used the bard, and for a brief time an inquisitor for skills, and they just took trap head on.

Healer - I would say this depends on the rest of the party makeup. My group had a barbarian, who took a lot of damage. no dedicated healer would have greatly shortened adventuring days. If you have a couple of classes that can do minor healing like Paladin, bard, Inquisitor, ect. you would probably be ok. I would think that books 2 and 3 would be a bit rough though.

Melee/ranged - At least 1 of each. If you are allowing leadership, Shelelu(sp) is a good choice for some ranged damage to the party.

I'm going to second Latrecis' call for a good healer. My party has both a Druid and a Warpriest, and they're still having difficulty addressing the incapacitating offenses they come across. Ability damage, blindness, paralyzation, confusion, polymorphing (this one messed them up GOOD last session), and more I'm forgetting are a real problem at high levels when you don't have a healer that preps for defending against that.

you could do it with a stealth group.( not that I would recommend trying it with a group of first time players... as I wouldnt)
bard, ninja, hunter, inquisitor, investigator, oracle with shadow mystery, cleric with shadow domian, rogue , monk and ranger.

Now my friend, i'm glad to help you, mostly because i did this question just one month ago, when i was about to start this campaing. Now, five sessions after the start, i can tell you one or two things that may help you.

1. Healers: trust me, your team WANTS a healer in the group. Clerics are usually the first choice, and maybe with an oracle they can manage. I say this because healing in the campaing is not only about the HP your team is going to loose but about the poisons and illnesses they will have to deal with. My group hasnt reach twistletop yet, and they have already been poisoned 3 times. Also, fear effects are pretty common, just like evil damage, and stat consumption.

2. Casters are allways usefull. This AP has a loooooot of history to discover, your characters can discover that history without a wizard, of course, but casters allways have a different vision and understanding of things. Also, the final tall folk of this campaing is...*SPOILER ALERT DONT READ THIS IF YOU ARE A PLAYER* a wizard himself. Any caster should fix the job, but wizards are the most suited for the task.

3. Rogues and ability classes are allways a good choice, yet you dont need one here, it will be easier and smoother to your group if they have one (damn, what kind of group refuses a man who can open any door without activating the trap?) however, a explorer, bard, or ninja can find a place in the group and manage to solve the lack of skill problem as good as a rogue would (only to the moment the team face a magic trap). If traps are a problem to them, tell the tank to go first XD.

4. My party are also four players, i would reccomend you 2 melee, 1 ranged and 1 caster. Just as Latrecis said, the game sistem rewards groups that cover every role. If your team is planning to let just one tall folk in the front, i would say two things to them.

First: the enemy is not stupid, if you put a tank in the front with 21 AC and they can avoid him to hit you, they are going to avoid him to hit you.

My group got all the way to level 13 without a cleric, they did have a lot of scrolls and a Paladin who could use most of them. They aquired a cleric at level 13 to replace a dead pc, but his death was not caused by the lack of a healer.
I do recomend a Wizard as it is really an AP which fits a wizard but you certainly can manage without one

I think any team can do the job if they got prepared (umd and scrolls can do anything) but everything is easiest with a cleric. Also everything is easiest with a wizard. This is cumulative.

With your newby team can be fun get a clasic 4 man rogue (unchained will be great, ranger or bard with disarm trap teait can do the job), wizard, cleric and a martial (unchained monk or barbarian will rock,a paladín is really solid but is honor coded, cavalier can work, fighter is nice but i dont like 2 skill points,etc)

Gonna run a metal oracle, and archer inquisitor, and a mutation warrior through this soon. Was considering adding a DM PC arcanist to hell them cover bases, but was thinking the arcanist could be a brown fur transmuter or something else very support focused to keep the spotlight on the players. Is this a good idea?

Edit: I should add these guys have never played pathfinder, and I fear making one play a wizard or arcanist would be too complicated.